This invention relates to catalysts. More particularly the invention relates to a method of making breakage resistant self-supported precipitated iron-based Fischer-Tropsch catalyst particles, to a method of making self-supported precipitated iron-based Fischer-Tropsch catalyst particles having superior synthesis performance or activity, to catalyst particles made according to the methods and to the use of said catalyst particles in a slurry bed Fischer-Tropsch reactor.
U.S. Pat. Nos. 5,324,335 and 5,504,118 disclose the production of roughly spherical iron-based Fischer-Tropsch catalyst particles having diameters in the range of between 1 and 50 microns which are annealed by heating in air at about 316xc2x0 C. (600xc2x0 F.) to drive off residual moisture and to stabilise the catalyst. The annealing step i.e. the heating and gradual controlled cooling, converts the Goethite to Hematite whereafter the catalyst may be activated and used. According to these patents, the annealing does not lead to a breakage resistant or a superior performance catalyst particle.
South African Patent No. 90/7530 discloses the production of an iron-based Fischer-Tropsch catalyst including from 1 to 80% by mass of activated carbon. This catalyst shows improved breakage resistance over conventional catalyst, particularly where the particle diameters are below about 45 micron. The catalyst particle of this patent does not have superior synthesis performance and is expected to hydrothermally sinter at about 300xc2x0 C.
A need thus exists for breakage resistant iron-based Fischer-Tropsch catalyst particles, in particular for use in a low temperature Fischer-Tropsch process, such as that carried out in a slurry bed reactor, for the production of, amongst others, wax and other syncrudes, as well as chemicals. The breakage resistant self-supported precipitated iron-based Fischer-Tropsch catalyst particles will ideally inhibit the formation of catalyst fines in the reactor thereby maintaining the performance of the reactor and reduce the contamination of down stream processes and catalysts by the catalyst fines.
In this specification, unless the context clearly indicates to the contrary, the term xe2x80x9cfinesxe2x80x9d when used in relation to catalysts and catalyst particles is to be understood to mean particles which due to their dimensions, when present at a concentration of about 30% of the total catalyst, tend to reduce the performance of the solid separation system of a Fischer-Tropsch slurry bed reactor. Typically fines have a diameter of less than about 45 microns, usually about 22 microns.
A further long felt need which exists is that for self-supported precipitated iron-based Fischer-Tropsch catalyst particles having superior synthesis performance or activity, in particular for use in a low temperature Fischer-Tropsch process, such as that carried out in a slurry bed reactor, for the production of wax and other syncrudes, as well as chemicals.
It is well expected that heat treatment of self-supported precipitated catalyst particles has a negative effect on the activity thereof. In particular, the catalyst particle surface area and pore volume are likely to be reduced at temperatures above 250xc2x0 C. Those skilled in the art therefore generally tend to avoid such heat treatment of such Fischer-Tropsch catalyst material.
Surprisingly it has now been found that the breakage resistance and the synthesis performance or activity of self-supported precipitated iron-based Fischer-Tropsch catalyst particles can be increased by the heat treatment thereof at temperatures of at least 250xc2x0 C.
Accordingly, the invention provides a method of producing self-supported precipitated iron-based catalyst particles for use in a Fischer-Tropsch slurry-bed process, the said particles being breakage resistant and thus inhibiting the formation of catalyst fines, the method including the heat treatment of the said particles at a temperature of at least 250xc2x0 C.
The heat treatment may be calcination of the said particles at a temperature of at least 250xc2x0 C.
The heat treatment of the said catalyst particles may be carried out at a temperature of between 250xc2x0 C. and 500xc2x0 C., preferably between 320xc2x0 C. and 500xc2x0 C., more preferably between 360xc2x0 C. and 390xc2x0 C., most preferably at 380xc2x0 C.
According to a second aspect of the invention, there is provided a method of producing self-supported precipitated iron-based catalyst particles for use in a Fischer-Tropsch slurry-bed process, the catalyst particles having a superior synthesis performance or activity under low temperature Fischer-Tropsch slurry-bed operating conditions, the method including the heat treatment of the said catalyst particles at a temperature of at least 250xc2x0 C.
The heat treatment temperature of the method may be between 250xc2x0 C. and 500xc2x0 C., preferably between 320xc2x0 C. and 500xc2x0 C., more preferably between 360xc2x0 C. and 390xc2x0 C., most preferably 380xc2x0 C.
Typically the said catalyst particles are maintained at the heat treatment temperature for at least 0.1 hours, preferably between 0.2 and 12 hours, more preferably between 0.5 and 4 hours.
According to a further aspect of the invention there are provided self-supported precipitated iron-based catalyst particles for use in a Fischer-Tropsch slurry-bed process, the said catalyst particles being produced according to a method of heat treatment of the said catalyst particles as described above.
According to yet a further aspect of the invention, there is provided a method of maintaining the performance of a solid separation system of a Fischer-Tropsch process slurry bed reactor where a reduction in performance is caused by an increase in catalyst particle fines in the slurry bed reactor, the method including the use of the catalyst particles as described above.
According to yet a further aspect of the invention, there is provided a process for synthesis of syncrudes and/or chemicals, for example, waxes, the process comprising the step of contacting a suitable synthesis gas, at suitable temperatures and pressures in a Fischer-Tropsch slurry-bed reactor, with self-supported precipitated iron-based Fischer-Tropsch catalyst particles as described above.
The process may be carried out in a suitable vessel, with unreacted reactants and gaseous product being withdrawn above the slurry bed, and separated liquid product also being withdrawn from the vessel.
Typical suitable operating temperatures for the process are temperatures in the range 160xc2x0 C. to 280xc2x0 C., or even higher for production of lower boiling point product.
Typical suitable operating pressures are pressures in the range 18 Bar to 50 Bar.